Cyanide is commonly used as a lixiviant to extract gold and other precious metals from ore. Milled ore is mixed with a liquid such as water to form a slurry or pulp, to which calcium cyanide or sodium cyanide is then added. An oxidizing agent is required for the dissolution of gold and other metals, and atmospheric air is the customary source of oxygen gas for use as the oxidant, although oxygen gas is also sometimes used.
A problem occurs, however, in ensuring that the oxygen is sufficiently diffused within the slurry for oxidation to occur so as to recover the maximum amount of gold from the ore. There is a strong resistance to the mixing of air or other oxidizing chemical agents with the slurry, which may have a consistency of 50% or more solids, and as a result only a portion of the air in the form of bubbles dissolves in the slurry to provide for oxidation.
The most commonly used method for injecting oxygen into the slurry is to use a lance/nozzle arrangement to inject air or oxygen into a tank or vessel containing an agitator. The shear from the agitation is then used to disperse the gas within the tank. A disadvantage of this method, however, is that the shear and Reynolds Numbers generated from the mixing action of the agitator are relatively low. Large bubbles are thus created, which tend to quickly flash off, resulting in low gas hold-up, low dissolved gas levels and low utilization efficiencies.
Other methods involve pumping the slurry through a pipe with back-pressure and one of a gas injection through a pipe or a lance, gas injection through slots, and gas injection through porous media, in each case relying on the turbulence within the system to break down the gas into bubbles.
Although these systems generally work better than gas injection into an agitated tank, they have several disadvantages. For example, relative gas hold-up and utilization efficiencies are still relatively low; there is high wear requiring frequent change outs; and the gas to be injected has to be pressurized to above the back-pressure of the system.
Another method involves the use of venturis or eductors, which create a measure of suction to draw in the gas into the slurry. Disadvantages, however, are that the system is not under pressure; larger bubbles are created which can flash off; and the gas hold-up and gas utilization efficiencies are relatively low.
There is therefore a need for an alternative apparatus and process for, inter alia, promoting the diffusion of a gas into slurry or for generating hydraulic cavitation in a process fluid.
In this specification, a fluid is to be regarded as including a liquid substance which may also comprise solid material, such as a pulp or slurry as well as entrained gas bubbles or even air. The liquid may be water or any other liquid, and the solid material may include milled or crushed ore, heavy metals, water contaminants, effluent, sewage, cellulose and so forth.